Democrats desperately want to retake the House in November, not just to thwart President Donald Trump's legislative agenda, but to launch countless investigations under the guise of oversight. However, their most stubborn obstacle to achieving this goal isn't President Trump or the Republican Party. The biggest obstacle actually sits inside their own party. Sen. Bernie Sanders (I-Vt.) has spent this cycle bankrolling primary challengers against sitting Democrats, and the fight has split the party into two camps that can barely stand each other.

Sanders recently announced plans to defeat a sitting House Democrat from North Carolina who belongs to the Congressional Black Caucus. House Democratic leader Hakeem Jeffries called Sanders personally to warn him off interfering in House primaries during a midterm year that could decide control of Congress, CNN reported, citing three Democrats familiar with the previously unreported call. Rep. Greg Meeks (D-N.Y.) sat in on that call and told the outlet that Sanders showed little concern about the warning. The call came just before Rep. Valerie Foushee defeated primary challenger Nida Allam in North Carolina in March. Jeffries later declined to describe the exchange beyond calling it honest.

Sanders sits at the height of his influence in Democratic politics right now, which makes the timing of this civil war particularly inconvenient for a party trying to project unity heading into the fall. The dispute comes down to a simple question: should Democrats run candidates who can actually win swing districts, or candidates who fire up the base and risk losing everyone else? Sanders has chosen one side, and the establishment the other. He has endorsed more than 90 candidates this cycle and helped topple House incumbents in New York City and Colorado. Of the 60 Sanders-backed candidates whose races have concluded, 42 have won, including 10 of 16 federal candidates. Senior Washington Democrats grumble that Sanders is forcing the party to burn money on internal fights instead of saving it for Republicans in November.

Then there's Graham Platner, the Maine Senate candidate Sanders backed and defended even after the New York Times reported allegations of abusive behavior toward women. Sanders kept defending him until after allegations of rape surfaced, which Platner has denied. Sanders withdrew his endorsement only then.

Platner's exit from the race, according to OpenSecrets, put the Democratic Party in a deep financial hole. Platner can't simply transfer his campaign funds to his replacement, and whoever takes his place will ultimately have to overcome a huge cash disadvantage compared to Sen. Susan Collins.

"Bernie has endorsed some flawed candidates recently," Rep. Hillary Scholten of Michigan said. "I think it's really important to look at the judgment that has been exercised. The Graham Platner situation should be a huge warning sign to a lot of voters about what that stamp of approval might mean."

Michigan is where this rift boiled over this weekend, as Sanders held rallies - joined by Rep. Alexandria Ocasio-Cortez - for Senate candidate Abdul El-Sayed and House candidate William Lawrence. Asked whether he's making it harder for Democrats to win in November, the 84-year-old Sanders said no and argued his candidates represent working people rather than wealthy donors. El-Sayed's support for Medicare for All and his criticism of Israel and AIPAC have made him a progressive favorite. At the same time, the party establishment favors Rep. Haley Stevens as the safer bet against likely GOP nominee former Rep. Mike Rogers. Retiring Sen. Gary Peters had planned to stay neutral in the August 4 primary but endorsed Stevens last week instead.

Senate Minority Leader Chuck Schumer has made his preference for Stevens clear. However, he avoided direct criticism of Sanders when asked, saying Democrats would settle on a nominee suited to the state. Rep. Ro Khanna compared Sanders' standing in the party to that of the pope and Barack Obama. Rep. Josh Gottheimer sees something else entirely.

"The DSA, which Bernie's part of, is trying to hijack the Democratic Party, and what you're seeing is people don't want that," Gottheimer said. "The Democratic Party wants to be the Democratic Party, and he's trying to use the Democratic Party as a vessel for socialism, and people don't want socialism."

Lawrence, a 35-year-old democratic socialist and climate activist, brings his own headache. Jeffries and Meeks condemned remarks reported by the Huffington Post in which Lawrence said black political leaders "take the teeth out of the white left" - language Jeffries called deeply disturbing - and party leaders worry a Lawrence win could cost them a must-win Lansing-area seat held by Republican Rep. Tom Barrett. Lawrence apologized on CNN, saying the comments misrepresented his values. The Congressional Black Caucus's political arm is funding ads against him, and Sen. Elissa Slotkin broke her usual neutrality to back former Navy SEAL Matt Maasdam, one of Lawrence's two rivals in the three-way primary along with former U.S. ambassador to Ukraine Bridget Brink. Even Ocasio-Cortez, scheduled to appear at the same Sunday stop, would not say whether she was comfortable campaigning alongside him: "I haven't endorsed in it. I haven't waded into it at all."